1. Field of the Invention
The present invention relates to an image reading apparatus and, more particularly, to an image reading apparatus having the function of correcting an image signal.
2. Related Background Art
In facsimile apparatuses, digital apparatuses, and the like, an original image to be transmitted or copied is photoelectrically read by means of a line sensor such as CCD or the like. The line sensor, in general, has thousands of photosensing elements, and separates the image of one line in to thousands of pixels and outputs an image signal indicative of light and dark of each pixel. In order to preferably read the image by such a line sensor, the output level of each photo sensing element for the images of the same density must be constant. However, if there is a variation in sensitivity or offset (dark current) of each photosensing element, the outputs of the respective photosensing elements are not uniform. In this case, for example, if a half-tone image is reproduced using the read-out signal, there occurs the inconvenience that black stripes appear in the reproduced image.
On the other hand, in the case of an apparatus in which the line sensor can independently output the image signal of the odd-number pixels and the image signal of the even-number pixels, the image processes such as amplification, digitization, and the like are executed by a different system for each of the two outputs.
In the foregoing apparatus in which the line sensor has two outputs which are powered by a respective plurality of systems, there is a case where the difference among the respective systems is larger than the variation among the pixels. Namely, since independent amplifiers, A/D converters, and the like are used for the outputs of the respective systems, a variation of offsets or the like of the respective systems occurs, causing the image quality to deteriorate. Such a difference can be removed by adjusting the gain of the amplifier of each system. However, the adjustment becomes very complicated and it is almost impossible to remove the time-dependent change.